Optical pressure sensor

ABSTRACT

A pressure sensor is formed on a substrate, the surface of the substrate having a p-n junction and a shell with a beam inside the shell over the p-n junction. The beam and the shell and the p-n junction surface form optical Fabry-Pérot cavities. An optical fiber is positioned in a hole formed in the underside of the substrate below the p-n junction. Light from the fiber charges the p-n junction and drives it into vibration mode. Pressure changes change the tension in the diaphram to vary the frequency with changes in pressure, so that pressure can be detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a pressure sensor, and inparticular to a resonant microbeam pressure sensor having a polysiliconmicrobeam resonator attached to a portion of the sensor diaphragm and aresonator beam and having an optical fiber directing a light beam to theresonator beam to read the resonance mode optically.

2. Description of the Related Art

A pressure sensor is disclosed in U.S. Pat. No. 5,808,210 in which amicromechnical sensor has a polysilicon beam that is an integral part ofthe diaphragm. The resonant frequency of the beam is a direct result ofthe pressure applied to the external surface of the diaphragm.Fabrication of this resonant microbeam sensor requires no backside waferprocessing, and involves a process and layout independent of waferthickness for high yield and robustness. Both the diaphragm and theresonant beam are formed from polysilicon. The sensor may have more thanone resonant beam. The sensor beam or beams may be driven and sensed byelectrical or optical mechanisms. For stress isolation, the sensor maybe situated on a cantilevered single crystal silicon paddle. The sensormay be recessed on the isolating die for non-interfering interfacingwith optical or electrical devices. The sensor die may be circular forease in mounting with fiber optic components. The contents of the U.S.Pat. No. 5,808,210 are incorporated herein by reference.

FIG. 2 of the present application sets forth an example of an earlierembodiment of a resonant beam pressure sensor. The FIG. 2 shows a thinfilm resonant microbeam absolute pressure sensor 10 in which a beam 12is held by posts 13 inside a shell 14. The shell 14 is provided on asubstrate wafer 16 with a vacuum, or at least a partial vacuum, insidethe shell 14. The substrate 16 has a photodiode, or p-n junction, 18formed on a top surface thereof within the shell 14. A Fabry-Pérotresonant cavity is formed within the shell 14, including a first portionbetween the beam 12 and the inside of the top of the shell 14 and asecond portion of the cavity between the resonant beam 12 and the top ofthe substrate 16. An optical fiber 20 which is positioned above theshell 14 at the front side of the sensor directs light onto the beam 12where it encounters the resonant cavity and resonates at a frequency.The resonating beam 12 reflects light back into the optical fiber 20which transmits the modulated light beam to a light sensor. Changes inpressure result in corresponding changes in the resonant frequency ofthe beam, so that the pressure is sensed. This device is termed a shellcoupled pressure sensor.

In the pressure sensor shown in FIG. 2, the light beam from the opticalfiber 20 is transmitted through the medium to be sensed, so that themedium under pressure must be transparent for the device to workproperly. If the medium is not transparent, some or all of the lightleaving the optical fiber 20 is scattered or absorbed before it canbounce off of the beam and re-enter the fiber. This results in degradedperformance of the sensor, or even to the point of the sensor beingunable to obtain a pressure reading at all.

A further problem is that of packaging the optical fiber 20 with thepressure sensor 10. The packaging must hold the fiber 20 close to theresonant beam 12 to get a good optical coupling; yet the spacing must besufficient that the medium to be sensed has access to the shell 14 ofthe pressure sensor 10.

In FIG. 3, the fiber 20 and sensor shell 14 are set at a fixed distancefrom one another by spacers 22. Openings 24 in the spacers 22 permit themedium to be sensed to access the sensor chamber 26 while maintainingthe spacing of the optical fiber 20 from the resonant beam 12. Thisarrangement addresses the issue of positioning of the fiber and sensor,but presents considerable difficulties in micromachining. Further, themedium to be sensed may become trapped in the sensor space 26 and canfoul the optical window at the end of the fiber 20. Any dirt or foreignmatter in the medium to be sensed may accumulate on the optical elementsof the sensor and fiber and result in deterioration in performance andeventually to complete loss of function.

SUMMARY OF THE INVENTION

The present invention provides a pressure sensor which is resistant tocontamination and which operates even in non-transparent media. Thepresent pressure sensor includes a substrate on which is mounted a shellforming vacuum resonant cavity within which is supported a resonantbeam. A p-n junction is formed on the top surface of the substrate belowthe resonant beam. An optical fiber carrying a light beam is in anopening in the underside of the substrate and is positioned to directthe light beam through the substrate toward the resonant beam. AFabry-Pérot cavity is formed within the shell to impose a frequency onthe light beam as it is reflected back into the optical fiber. Thesensor is thus able to provide a frequency output that is a directmeasure of the pressure applied to the top surface of the shell. A thinfilm resonant microbeam absolute pressure sensor results.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a pressure sensor according to theprinciples of the present invention;

FIG. 2 is a cross sectional view of a pressure sensor of an earlierembodiment; and

FIG. 3 is a cross sectional view of a pressure sensor of the earlierembodiment showing spacers between the optical fiber and the sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an improvement on the earlier embodimentsof the thin film resonant microbeam pressure sensors. In particular, apressure sensor 30 is provided having a resonant beam 32 held by posts34 in a shell 36. The shell 36 is mounted on a substrate 38 in the topsurface of which is provided a photodiode or p-n junction 40. Theinterior of the shell 36 is evacuated to provide at least a partialvacuum. A Fabry-Pérot optical resonant cavity 42 is formed in the shell36 including a first portion between the beam 32 and the inside topsurface of the shell 36 and a second portion between the underside ofthe beam 32 and the top surface of the substrate 38. The beam 32 may bepartially transparent to permit light to move between the two portionsof the optical cavity, or they may be opaque in which case only thelower Fabry-Pérot cavity is optically active. The shell 36 and beam 32are preferably of polysilicon and constitute a microstructure.

An optical fiber 44 is provided in an opening 46 in the underside of thesubstrate 38. The optical fiber 44 is mounted to direct the light beamfrom the fiber 44 toward the resonant beam 32. The opening 46 in theunderside of the substrate 38 permits the fiber 44 to be positionedclose enough to the beam 32 to permit the sensing of the light reflectedby the beam 32.

To form the opening 46 in the substrate 38, a silicon on insulator (SOI)wafer is provided having a layer of silicon 48 of about 10 micronsformed over a layer of silicon dioxide (SiO₂) 50 of about 1 micron inthickness. The silicon substrate 38 is generally several hundred micronsthick. The opening 46 is formed by first photo-patterning a circularhole on the back side of the substrate wafer 38 and then etching thesubstrate material. The circular hole is preferably slightly larger thanthe diameter of the optical fiber 44. The etching process stops at or inthe silicon dioxide layer 50, leaving the thin silicon layer 48 at thetop surface of the substrate 38 at the hole. The selectivity of theetchant for silicon to silicon dioxide is about 300:1, so the etchprocess is relatively easy to stop at the silicon dioxide layer.

The etching step may be performed before the formation of the sensorstructure on the substrate, during the formation of the sensor structureon the substrate, or after the formation of the sensor structure on thesubstrate.

A hole 46 is etched all the way up to the silicon dioxide layer 50 andthe optical fiber 44 is positioned in the hole. Since the silicon layer48 and the silicon dioxide layer 50 are both transparent to infra-redlight, an infra-red light source 52 is used so that the light from thefiber passes through the silicon 48 and silicon dioxide 50 layers andreaches the photodiode 40 and the beam 32 and shell structure 36.

In operation, the infra-red light from the optical fiber 44 travelsthrough the silicon dioxide and silicon layers 50 and 48 and into thecavity 42 within the shell 36. The light is partially reflected by thesurfaces of the beam 32, the surface of shell 36 and the surface of thephotodiode or p-n junction 40. The optical cavity 42 provides multipleoptical paths involving the top of the photodiode 40, the resonator beam32 and the inside surface of the shell 36. The various optical pathsoperate as a set of Fabry-Pérot cavities. The various beams of lightthat travel along the various optical paths will either constructivelyinterfere with one another or destructively interfere with one anotherwhen they arrive at the photodiode 40. The intensity of light arrivingat the photodiode therefore changes as the spacing between the resonatorelements changes.

Light energy striking the p-n junction 40 at the surface of thesubstrate 38 results in a charge accumulation in the p-n junction 40.The charge accumulation draws the resonator beam 32 toward the p-njunction 40 through electrostatic attraction. If the light energystriking the p-n junction 40 is reduced, the beam relaxes back away fromthe junction. If the light energy striking the p-n junction 40 isincreased again, the beam is attracted to the junction again. Themovement of the beam 32 continues in a cyclic manner to create aphysical resonating movement, or vibration, of the beam. The lightenergy striking the p-n junction may be increased and decreased eitherby modulating the light energy at its source (e.g. modulating theintensity of the laser), or by making the structure self-resonant astaught in U.S. Pat. No. 5,559,358.

The physically resonating beam 32 in the optical Fabry-Perot cavitiesmodulates the light that is reflected back into the optical fiber withchanging frequencies. The modulated light is sensed by the sensor 52 todefine a pressure state of the pressure sensor. When the sensor element30 is subjected to pressure changes, the tension on the beam changeswhich changes the frequency of vibration of the beam and a correspondingchange in the modulation of the light returned into the fiber 44.Therefore, pressure changes are sensed in the present pressure sensor30.

It is noted that no electrical connections are present to the photodiodeor p-n junction 40. The junction is present to accumulate charges.

The present invention effectively senses pressure even though the lightof the optical fiber 44 passing through the p-n junction 40 and thesubstrate 38. In particular, with the present invention it is stillpossible to sense the modulation of the light and the variations thereineven though the p-n junction 40 is directly illuminated by the light ofthe optical fiber 44 passing therethrough. This is because the lightthat passes through the Fabry-Perot cavity after illuminating thephotodiode interferes with the light that directly illuminates thephotodiode.

By positioning the optical fiber 44 in the hole 46, it is possible tolocate the fiber 44 about 10 microns from the resonant beam 32,resulting in a good optical coupling between the beam 32 and the fiber44.

The present invention permits the sensing of pressure using a resonantbeam sensor without regard to the optical properties of the medium to besensed. The medium may be dirty or even opaque. Furthermore, the mediumdoes not result in contamination of the optical surfaces.

The present invention permits the use of a simple process for sensormanufacture. No spacers are required to define the beam/fiber spacingand no pressure access channels need to be formed. Further, no spaces totrap dirt or fluids are present.

Manufacturing costs are low and simple to carry out. The sensor element30 is simply bonded directly onto the optical fiber 44.

The die for the sensor 30 may be very small, and as such is cheap tomake. The hole 46 for the fiber may be made by deep reactive ion etchingwhich is only a few hundred microns wide.

The present sensor may be made extremely small and may have a diameteronly slightly larger than the tip of an optical fiber.

Although the present application calls for a beam as the resonatingmember, it is also envisioned that the resonating member may be of avariety of shapes, including a disk, for example.

Thus, there is shown and described a pressure sensor utilizing anoptical beam to sense changes in resonant frequencies of opticalresonators which modulate the light by physical resonance.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

1. A method for sensing pressure, comprising the steps of: directing abeam of light through a junction in a semiconductor substrate and onto abeam; providing an optical resonant cavity between said junction andsaid beam to impose a frequency on said beam of light; vibrating saidbeam by electrostatic attraction between said junction and said beam;varying a frequency of vibration by said beam by changes in pressure toeffect changes in said beam of light; and sensing said changes in saidbeam of light.
 2. A method as claimed in claim 1, further comprising thestep of: providing a evacuated chamber within which said beam ismounted, said evacuated chamber having an outer shell defining anoptical resonant cavity between said outer shell and said beam.